Fastening portion structures of resin parts are known in the art in which the resin part has a tubular member integrally embedded therein by insert molding, and has an insertion hole formed inside the tubular member so that a screw member is inserted therethrough, and the tubular member is compressed in the axial direction by the screw member, whereby the resin part can be fixed to a predetermined member (see Patent Document 1). A fastening portion 100 in FIG. 12 is an example of such a fastening portion structure. A resin part 102 has as the tubular member a cylindrical collar 104 integrally embedded therein by insert molding. The collar 104 has a plurality of through holes 106. A synthetic resin has flown into the collar 104 through the through holes 106, and the resin 108 inside the collar 104 has an insertion hole 110 formed substantially concentrically with the collar 104 so that a fastening bolt 112 as the screw member is inserted therethrough. The fastening bolt 112 inserted through the insertion hole 110 is screwed into a threaded hole 116 formed in a predetermined attachment member 114, and the collar 104 is compressed between a head of the fastening bolt 112 and the attachment member 114, whereby the resin part 102 is fixedly attached to the attachment member 114 via the collar 104. The size (radial dimension) and shape of the insertion hole 110 can be set as desired independently of the collar 104 when insert molding is performed.